We've unlocked the Underhold wing of the Siege of Orgrimmar (Malkorok, Spoils of Pandaria, and Thok the Bloodthirsty) for testing over the weekend, continuing into Monday.

Players with a premade group of between 10 and 25 players can queue using the same interface that you'd use for Dungeons, Scenarios, or LFR. Flexible mode bosses have all the same mechanics as the Normal mode versions, but are otherwise tuned to be more forgiving, and will scale in health and damage output with the number of players in the raid. As you go from a 12-player raid to an 18-player raid, for example, some abilities may affect more targets or hit slightly harder, creatures will have more health, and so forth. Difficulty is almost certainly higher than our final target, but we're probably scaling players down a bit too far for testing right now (ilvl 522 for the Underhold wing -- 530 is a probably the right target there, but hotfixing that may not happen until after the weekend).